I am writing this column in Japan, a country whose crime rate is the lowest among countries with large populations. I asked my Japanese translator, a middle-aged woman, what she thought.

"Why is there is so little crime in Japan?" I asked.

Without taking a moment to reflect, she responded, "Because we don't allow immigration."

Anyone who visits Japan is struck by the ethnic homogeneity of the nation. If you meet a Caucasian, a black or a Hispanic in Japan, you can be all but certain that the person is visiting or studying there, not a citizen.

Likewise in the United States, there is direct correlation between ethnic homogeneity and low levels of violence. According to 2016-2017 data, the four states with the lowest percentages of violence are:

1. Vermont — where 95 percent of the population is one race (white).

2. Maine — where 95 percent of the population is one race (white).

3. Wyoming — where roughly 93 percent of the population is one race (white).

4. New Hampshire — where roughly 94 percent of the population is one race (white).

Sweden, which for much of its modern history has had among the world's lowest rates of violent crime, was almost always as homogenous as Japan. Now that it has admitted hundreds of thousands of immigrants from the Middle East and North Africa, it is no longer a homogenous country, and its levels of violence have increased dramatically.

All this leads to a particular rule, which is, in order to maintain a low crime rate and social stability, a country has only two choices: Do not allow immigrants into the country, or allow immigrants into the country, but be certain to assimilate them into the native population as quickly as possible.

The second choice has been America's choice throughout most of its history, and it has been uniquely successful in shaping people from all over the world and from every background into one nation known as Americans. One of America's three fundamental principles has been e pluribus unum, or "out of many" (the other two, as our coinage testifies, are liberty and In God We Trust). And that is precisely what America has done.

But since the 1960s, the left has supplanted e pluribus unum and its national American identity with the antithetical doctrines of diversity and multiculturalism.

Diversity and multiculturalism celebrate the national/ethnic identities of the nations from where American immigrants came instead of celebrating the American identity and traditional American values.

The result is the beginning of the end of the United States as we have known it since its inception.

The left constantly repeats "we are a nation of immigrants" without citing the other half of that fact — "who assimilate into America." The left mocks the once-universally held American belief in the melting pot. But the melting pot is the only way for a country composed of immigrants to build a cohesive society.

America was never just "a nation of immigrants." America was always a nation of immigrants who sought to become — or at least were taught by American public schools and by the general American culture to become — Americans.

If America becomes a nation of nonassimilating immigrants, or a nation consisting of nonassimilating ethnic, racial and national groups who are already here, it will cease being a glorious idea and become just another nation torn by conflicting interest groups. These various groups will fight one another — first verbally and then, perhaps, violently (and America will see more and more violence) — just as France, Sweden and Germany have seen since they began taking in millions of immigrants, many of whom have no intention of becoming Frenchmen, Swedes or Germans.

Contrary to one of the left's more mendacious claims, diversity has not been America's great strength. America's great strength has been forging an American identity out of diversity.

But the left, with its identity politics and commitment to multiculturalism — as expressed, for example, by ballots in dozens of languages, the proliferation of ethnic studies departments at universities and the allowance of all-black dorms and graduation ceremonies — is undoing that.

If you want to understand the immigration crisis, just know that because the left has undone the second choice, it has made the first choice — Japan's choice — look tenable to many for the first time in American history.